To control an operation of, for example, a walking aid apparatus for assisting a human being in walking, it is necessary to grasp the joint moments actually acting on joints of legs of the human being. Grasping the joint moments makes it possible to properly determine desired auxiliary forces of the walking aid apparatus. In a bipedal walking robot also, there are cases where it is required to grasp a joint moment actually acting on each joint of a leg to conduct the operation control.
Hence, the present applicant has previously proposed in, for example, Japanese Unexamined Patent Application Publication No. 2003-89083 (hereinafter referred to as Patent Document 1), a technique for estimating a joint moment of a leg of a bipedal walking body, such as a human being. According to this technique, a displacement amount (rotational angle) of each joint of a leg of the bipedal walking body, the acceleration of a predetermined part, and angular velocity are measured using required sensors, and then the measurement data and a rigid link model of the bipedal walking body or the like are used to estimate a floor reaction force (translational floor reaction force) acting on each leg and the position of an acting point thereof. Here, the rigid link model is a model representing a structure of the bipedal walking body in terms of an assembly formed by connecting a plurality of rigid elements by a plurality of joint elements. The rigid link model is used to estimate the position of the overall center-of-gravity of the bipedal walking body, the positions and the postures of corresponding rigid bodies of the bipedal walking body (thighs, cruses, waist, etc.) respectively associated with the rigid elements and the joint elements, and the positions and the postures of joints (knee joints, hip joints, etc.), and it is also used as the basis of the model for describing a dynamic behavior of the bipedal walking body. In the rigid elements of the rigid link model, the weights, the lengths, and the positions of the centers of gravity thereof (the positions on the rigid elements) are collaterally set beforehand.
And, according to the one in Patent Document 1 mentioned above, the estimated floor reaction force and the position of the acting point thereof and the rigid link model are used to estimate the joint moments of the knee joints or the hip joints of the legs by arithmetic processing based on an inverse dynamic model. The inverse dynamic model is generally described as a dynamic model for estimating a reaction force or a moment, which is an internal force of an object, an external force acting on the object and positional information being known (the external force and the positional information being input parameters), and it represents the relationship between motions of the object (a positional time-series pattern) and forces or moments acting on the object. According to the technique in Patent Document 1 mentioned above, the inverse dynamic model is constructed on the basis of dynamic equations related to motions (translational motions and rotational motions) of the rigid elements of the rigid link model, and joint moments of each leg are estimated in order, the moment of the joint closest to the acting point of a floor reaction force being the first to be estimated.
When assisting a human being as a bipedal walking body with walking, it is desired to estimate the joint moment of each joint with high accuracy especially in the direction in which a leg bends or stretches in order to precisely perform the assistance of the walking. For this reason, according to an embodiment of the one in the Patent Document 1 mentioned above, the motion (two-dimensional motion) of a human being on the plane (sagittal plane) of a vertical posture with the lateral direction of the human being defined as a normal line direction is grasped so as to estimate a joint moment (a moment about a lateral axis).
However, joints, such as a hip joint, of a leg of the human being are capable of three-dimensional (spatial) motions, including the motions of a leg in the bending and stretching directions, and they are capable of a motion of, for example, moving each leg in the lateral directions by rotation about a substantially longitudinal axis of a hip joint (so-called abduction and adduction) or a twisting motion (turning motion) of each leg by rotation about a substantially vertical axis of the hip joint. Therefore, there are many cases where bending motions of the legs are not performed on the sagittal plane of a vertical posture when a human being moves. In such cases, there has been likelihood that the accuracy of estimating joint moments in the bending/stretching directions of the legs deteriorates in an embodiment of Patent Document 1 mentioned above.
Considering the three-dimensional motions of the legs described above, in order to accurately estimate the joint moments of the legs of a bipedal walking body as much as possible, it is considered desirable to grasp the motions of parts of the bipedal walking body (the positions, postures, accelerations, and the like of corresponding rigid bodies) and the floor reaction force acting on each leg of the bipedal walking body and the position of the acting point thereof in terms of three-dimensional amounts (a set of coordinate component values in a given three-dimensional coordinate system) so as to estimate a joint moment of a leg on the basis of the grasped values.
However, in this case, it is necessary to grasp, using appropriate sensors, the three-dimensional displacement amounts of the hip joints, the knee joints and ankle joints of the legs. And, in particular, the joints of the legs of the human being as a bipedal walking body are capable of performing complicated motions and are apt to be subject to restrictions on the mounting locations or mounting forms of the sensors for detecting the displacement amounts. For this reason, it is usually difficult to grasp every component of the three-dimensional displacement amounts of the joints with sufficiently high accuracy. Further, the accuracy of the displacement amount of a joint grasped from an output of a sensor tends to vary, depending on a posture state or the like of a leg.
Hence, even if an attempt is made to estimate a joint moment in the bending/stretching directions of a leg by simply using a three-dimensional technique, there has been a danger in that an error inconveniently increases or an estimated value thereof inconveniently tends to suddenly changes (the robust performance deteriorates).
The present invention has been made in view of the above background, and it is an object of the present invention to provide a method of estimating a joint moment of a bipedal walking body that permits enhanced stability of an estimated value of a joint moment in bending/stretching directions of a leg, while securing estimation accuracy, considering three-dimensional motions of the bipedal walking body.